Air-conditioning units are known which are connected between sets of riser pipes and which allow the temperature adjacent each unit to be controlled individually without affecting the operation of the other units connected to the same riser pipes. Such units include a heat-exchanger coil connectable to the riser pipes through thermostatically controlled motorized or solenoid valves, and a blower for forcing air through this coil at a steady rate. As a general rule the fan runs continuously to circulate the air, and even filter it, while the temperature of the coil is varied. It is possible to use a so-called two-pipe system for both heating and cooling by providing a closed-cycle refrigerator unit in the device which has a condenser coil cooled by the water which is simultaneously hot enough to be used for heating in an adjacent unit. Otherwise in a two-pipe system the standard procedure is simply to run chilled water through the pipes in the summer and hot water in the winter, with suitable controls on the thermostat for the seasonal switchover. In four-pipe systems use is made of motorized zone valves which allow one unit to be used for heating while another is used for cooling.
Such devices present considerable advantages, especially in rental buildings. Each tenant has control of his own heating and/or cooling which is very desireable especially in commercial structures where, for example, machine-filled rooms might need air conditioning practically year-round while store-rooms might be left totally unheated and uncooled for economy reasons, at the option of the tenant.
A considerable disadvantage, of such a system is that when a unit fails it often requires considerable technical skill to repair it. The entire system must be shut down to allow the unit to be serviced, and even then the person in control of the unit is frequently left without heating or cooling for a protracted period until the repair person can do the job.